(1) Field of the Invention
The present invention relates to microwave circuit boards and a method by which such microwave circuit boards are produced. Generally speaking, a microwave circuit board comprises two conductive foils having inserted therebetween a material having a predetermined dielectric constant. The microwave circuit boards are processed to remove portions of the conductive foil to define a circuit pattern determined by the intended use. The removal of parts of the foil is usually accomplished by etchants, solvents, or the like.
(2) Description of the Prior Art
One type of prior art microwave circuit board is disclosed in a paper presented at IEEE/NEMA 1975 Electrical Electronics Insulation Conference at Boston, Mass. on Nov. 11, 1975, "EPSILAM 10", a product of the 3M Company, is believed to be a composite of poly(tetrafluoroethylene) (PTFE) and dielectric filler, the composite typically having a dielectric constant of between 10 and 11 and being clad on both sides by copper foil.
In general, prior art microwave circuit boards exhibit numerous disadvantageous properties. For example, prior art microwave circuit board tend to absorb moisture to an undesirable degree. Thus, when the conductive foil of the microwave circuit boards is removed by etchants or solvents, the underlying dielectric material absorbs moisture in undesirable amounts. The absorption of moisture results in at least two serious problems: the electrical properties of the circuit board are changed and the circuit board may physically expand, thus straining tolerances which must be strictly observed in microwave circuit design.
The prior art circuit boards, when processed to remove portions of the conductive foil to define circuits, tend to undergo strain relief to an undesirable degree. Strain relief is a measurement which quantifies the dimensional changes in the microwave circuit board after it is processed to remove a portion of the conductive foil. As will be understood by those skilled in the art, it is undesirable to have a circuit board which changes in shape substantially when processed to remove a portion of the copper foil.
Moreover, prior art microwave circuit boards tend to have a non-uniform dielectric constant. It is believed that this non-uniformity of dielectric constant is due, at least in part, to incomplete mixing of the dielectric filler with the PTFE. Thus, in certain portions of the circuit board, there may be an excess of PTFE and insufficient dielectric filler.
Furthermore, prior art microwave circuit boards tend to have a relatively high energy dissipation factor.
Another type of prior art microwave circuit board is produced by Rogers Corporation, Rogers, Conn., and marketed under the trademark "RT/duroid". This product, which comprises PTFE and glass microfibers, has the desired strain relief properties but does not have a sufficiently high dielectric constant for some applications.
It is an object of the present invention to provide a microwave circuit board having a relatively high dielectric constant, said dielectric constant being uniform throughout the circuit board.
It is another object of the present invention to provide a microwave circuit board which is moisture-resistant, that is, the circuit board resists absorption of moisture.
It is yet another object of the present invention to provide a microwave circuit board having a relatively high tensile strength and an acceptable dissapation factor.
It is a further object of the present invention to provide a microwave circuit board having improved strain relief properties.